In recent years, attention has been focused on a technique for forming transistors using semiconductor thin films formed over a substrate having an insulating surface. The transistors are applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). As materials of semiconductor thin films applicable to the transistors, silicon-based semiconductor materials have been widely known, but oxide semiconductors have been attracting attention as alternative materials.
For example, disclosure is made of a transistor having an active layer for which an oxide semiconductor that contains indium (In), gallium (Ga), and zinc (Zn) and has an electron carrier concentration less than 1018/cm3 is used, and a sputtering method is considered the most suitable as a method for forming a film of the oxide semiconductor (see Patent Document 1).